Upgrade was not successfull. Your previous version of windows is being restored.

Well assuming you have purchased an upgrade key then you are not allowed to have the two OS at the same time.

Personally I would do the clean install via the Dell BIOs (full instructions as in my guide here). If your media is not bootable then you can download the appropriate Windows 7 .iso from here and create a bootable DVD or USB flash drive.

There are many routes to proceed. You may opt not to format the drive and instead extract all your old files from Windows.old, this is not the route I recommend and I will instead walk you through what I would personally do.

Install your second 1 TB hard drive and create the following folders on it:

• Documents - Copy all you want from documents to it
• Musics - Copy all you want from music to it
• Pictures - Copy all you want from pictures to it
• Videos - Copy all you want from Videos to it
• Downloads - Copy all you want from Downloads to it
• Favourites - Copy all you want from Favourites to it*
• Desktop - Copy all you want from Desktop to it

*Copy from C:\Users\"Your Name"\Favorites

I put these all in a folder called Backup (date)

Next run Belarc Advisor as this will  will obtain details about your system. It is free for home use and gives you a print out of your computers hardware and software. It also gives you your product keys. This is particularly useful for obtaining your Windows Vista (likely to be a Dell volume license key - also confer with COA) and Microsoft Office product keys. Use CutePDF Writer to create a pdf print out of it and save the PDF to a folder called Computer Software on the second hard drive.

Next you will need to obtain all you need:

First you need to see if your Windows 7 DVD includes Service Pack 1 (assuming you have a DVD). If it doesn't then download it Service Pack 1 (windows6.1-KB976932-X64.exe) and also download Internet Explorer 9 (x64). Save these in a folder within Computer Software and call it 1. Windows & Windows Updates. In this folder create a notepad file called Product Key, in this file type in your Windows 7 Product Key for Backup Purposes.

If your media is not bootable then in this folder save the correct .iso file from here and the Microsoft .iso to USB/DVD tool. 

Next if you have Microsoft Office, download the correct version from here and latest Service Packs (Slipstream them into it if you want - optional). Save these into a folder called 2. Microsoft Office (within Computer Software). In this folder create a notepad file called Product Key, in this file type in your Office Product Key and Version. See example below (Note my structure is slightly different):

Next Create a folder called 3. Dell Drivers, download the latest drivers for your system here. See the clean install of Windows 7 guide guide for more information about what ones to download, how to make it specific for your system and what order to save them in.

Make a folder structure such as the one for my Inspiron 6400 within this (expanded folders to the left). Make sure you have everything named so you know what is what and have them in a numeric structure for the order you will/should install them in. (Again see the guide for more details).

Now create a folder 4. Security (within Computer Software). Then within this create a folder called Microsoft Security Essentials, download Microsoft Security Essentials (64 bit) and Latest Security Definition (64 bit). Also download Malwarebytes' Antimalware and Sphinx Windows 7 Firewall Control, likewise save it in its folder. This is optional (assuming you are going via my software suggestions as here). Preinstalled software for some newer systems is available to download from My Dell Downloads*. Here the user should find the preinstalled McAfee** should they wish to use it instead.

You may download any other software from My Dell Downloads (if you have this service - not sure if it applies to your system) and save it in the Dell folder or where you deem appropriate. Likewise you may want  Dell Support Centre, preinstalled Skype, Dell Webcam Software & Dell Dock.

You may download and save any other software you wish to install in a folder you deem appropriate on the second hard drive.

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The above was all just house keeping to ensure you have a backup of all your files and have all the installation files at your finger tips. Now for actually installing Windows 7.

Insert the DVD you have created or Dell reinstallation DVD or the bootable USB flash stick. Power down your computer. Hold F12 while powering up your computer (at the Dell BIOs screen).

Select boot from CD/DVD or from USB respectively. Next I refer you to here. Pay attention to step 6. and ensure you click the custom (advanced option). Pay attention at Step 7. pick the partition where Vista is installed, select drive options and then format. Continue installing Windows 7. The rest should be self-explanatory (on screen).

Then install the Service Pack 1 (if necessary and Internet Explorer 9). Next the Dell system drivers (see the guide for installing these). Then install the office and the security and then connect to the internet. Note the firewall I recommend blocks everything by default (except windows update) so you will need to tell it to allow programs to access the internet, it will prompt you when a new one is trying to access the internet.  Install other desired applications and copy the documents and everything back other.